1. Field of the Invention
The present invention relates to an active noise reduction device for reducing the noise produced in an electronic apparatus such as a computer.
2. Description of the Related Art
An electronic computer comprises a casing (cabinet) and various electronic components arranged in the casing. The electronic components usually produce heat and the temperature in the casing rises. A cooling fan (or cooling fans) is thus arranged in the casing to convey heat to the outside of the casing. In particular, fresh air is sucked into the casing via an air inlet, and forcibly blown through the casing by the cooling fan to cool the electronic components, the hot air being exhausted from the casing via an air outlet. In such an apparatus, the cooling fan produces noise which leaks i.e., is transmitted, to the outside of the casing via the air inlet and the air outlet. Therefore, the working environment in an office in which the computer is used is affected adversely.
To actively reduce noise, it is known to use an active noise reduction device in a structure. The active noise reduction device can reduce a noise by making a sound having a waveform with the same amplitude as that of the noise but with the opposite phase of the noise, the sound being superposed on the noise. In the conventional active noise reduction device, the noise is introduced into a special duct. A microphone is arranged in the duct near the inlet thereof for detecting the noise. A digital filter generates a noise reducing signal in response to the output of the microphone, and a speaker is arranged in the duct near the outlet of the duct. The speaker makes a sound based on the noise reducing signal and the sound has a waveform with the same amplitude as that of the noise but in the opposite phase of the noise. The sound is superposed on the noise propagating in the duct to thereby reduce the noise.
The conventional active noise reduction device, for example, is built in an air conditioning system including a duct extending along the ceiling of a building structure for reducing a noise induced by a flow of conditioning air in the air conditioning system. In this case, an additional duct is not necessary for the active noise reduction device because the duct of the air conditioning system can be also used for the active noise reduction device. However, when the active noise reduction device is intended for use with a particular apparatus, it is necessary to arrange a special duct for the active noise reduction device in the apparatus. Therefore, the overall size of the apparatus becomes larger and applications of the active noise reduction devices are limited.
In order to reduce a noise in a computer, it is possible to arrange an active noise reduction device within the computer, but this concept has not been realized since the active noise reduction device includes a duct which is to be incorporated in the casing of the apparatus along the path of the cooling air flowing in the casing. Conventionally, attempts to reduce noise in a computer have been directed to the design of the cooling fans so that the cooling fans themselves have a quiet structure. However, recent computers must be highly efficient and include many electronic components and a plurality of cooling fans for cooling the electronic components, so that the produced noise is high. In this situation, attempts to reduce noise by the design of the cooling fans cannot satisfy the requirements, and thus a further reduction in the noise is desired. Accordingly, the present invention aims to mount an active noise reduction device in a computer.
In some cases, it is desired to mount an active noise reduction device in a computer, for example, when the computer is arranged in a quiet office, and in some cases, it is not necessary to mount an active noise reduction device in a computer, for example, when the computer is arranged in a relatively noisy factory. If the active noise reduction device is fixedly incorporated in the computer, it is not easy to remove the active noise reduction device from the computer when the active noise reduction device is not necessary. Therefore, preferably, an active noise reduction device can be easily mounted in, and removed from, a computer. Also, it is desired that an active noise reduction device can be mounted on an existing computer.